Injection molding is a well-known process used for the fabrication of plastic or metal objects or parts having complex shapes. In the injection molding process, a molten material, such as a plastic or a metal, is introduced into a mold and allowed to set or cure by cooling. Once the molten material is set or cured, the mold is opened, and the molded object is released. The temperature of the mold is preferably controlled throughout the molding process, for example to ensure the quality of the molded object, and to maximize production throughput.
Proper temperature control is important when the molten material is injected into the mold, for example to avoid problems such as incomplete fill, poor part weld, and excessive stress in the part. The optimal mold temperature specified by the material manufacturer is typically well above room temperature, so proper temperature control usually requires heating the mold before introducing molten material into the mold.
Various methods can be used to heat injection molds to an optimal temperature before introducing molten material. For example, the mold can be heated simply by introducing molten material. This approach may necessitate a startup cycle wherein the first few molded parts contain defects, for example because of incomplete fill, and these defective parts must be discarded or recycled until the mold reaches an adequate temperature to produce parts that are free of defects. Another disadvantage of using molten material to heat the mold is that the mold must keep molding parts without interruption, or the mold will cool and the startup cycle and its associated waste must be repeated. Depending on the work environment, this startup cycle could be needed every morning, lunchtime, or coffee break. Similarly, variations in the delay between molding of successive objects can result in variations in the temperature of the mold when the successive objects are molded, which may reduce the uniformity of the successive objects.
External heat sources can also be used to heat the mold to an optimal temperature, for example electric heating elements or heated fluid can be used. These external heat sources can be applied to the mold in a variety of ways known in the art. External heat sources can avoid the required startup cycle and waste associated with using molten material to heat the mold. This approach can also ensure that the mold temperature remains consistent across the molding of successive objects, thereby improving the uniformity of the successive objects.
To produce high quality molded parts, optimum temperature control may require the application of heat in a non-uniform fashion, both across the area of the mold and over time. Heat can be applied at the periphery of the mold to heat the entire mold to a temperature which is essentially uniform across the molding surface, or applied in a non-uniform fashion to specific portions of the molding surface, for example the extremities of the mold cavity that may be the areas most likely to experience problems such as incomplete fill. Heat can be applied continuously over time, as a pulse of heat at a certain point in the mold cycle, or in a time-varying fashion.
Proper temperature control is also important after the molten material has been injected into the mold, during the period of time when the molded object sets or cures. When hot molten material is injected into a mold, the mold absorbs heat from the molten material and the temperature of the mold will increase toward the temperature of the molten material being injected into the mold. Thus, after molten material has been injected into the mold, it is desirable to provide cooling to remove heat from the mold and the molten material contained in the mold so that the molded object will set or cure, for example, to improve the quality of the objects being molded or to increase the productivity of the mold.
As with heating, optimum temperature control may require the application of cooling in a non-uniform fashion, both across the area of the mold and over time. Cooling can be applied at the periphery of the mold to cool the entire mold uniformly, or applied in a non-uniform fashion to specific portions of the mold, for example, the hottest areas of the mold such as thick portions of the mold cavity that receive a relatively large volume of molten material, portions near the injection channel for molten material that receive molten material that is relatively hot, or portions adjacent to heating elements. Cooling can be applied continuously over time, as a pulse at a certain point in the mold cycle, or in a time-varying fashion.
Proper temperature control after the molten material has been injected into the mold can affect the quality of the molded parts in a number of ways. For example, it is generally desirable to control the eventual temperature and cooling rate so that the plastic or metal object being molded exhibits the least possible amount of shrinkage and distortion during the setting or curing process. It is also important to control the application of cooling so as to ensure uniformity among replications of the object being molded.
In addition to improving the quality of the molded objects, proper temperature control can maximize productivity of the mold. For example, to minimize the setting or curing time after molten material has been injected the mold should be quickly cooled to an optimal temperature for setting or curing the object being molded, at the maximum rate possible which will nonetheless result in a molded object of acceptable quality. Similarly, after a first object is molded and removed from the mold, the mold should be heated quickly to an optimal temperature for receiving a new injection of molten material to form a second object, at the maximum rate possible which will not damage the mold or otherwise adversely affect the molding process.
Temperature control of an injection mold has been accomplished by circulating fluid through channels fashioned in the walls of the mold. In such a system, fluid is heated and then circulated through the mold to heat the mold to an optimum temperature before the first injection or “shot” of hot plastic or metal material is introduced into the mold. Because there is good thermal conductivity between the mold and the fluid, the temperature of the mold will be close to the temperature of the fluid until molten material is injected into the mold. The optimum temperature of the fluid and the mold is usually well above room temperature but below the temperature of the hot molten material.
Upon the introduction of the hot molten material, the temperature of the mold increases above the temperature of the fluid. The temperature of the fluid, however, is maintained at the optimum temperature, for example using an external heat exchanger or chiller. The continuous circulation of the fluid removes heat from the mold, thereby returning the temperature of the mold (and the molten material forming the object being molded) to a temperature at or near the temperature of the fluid so that the object being molded sets or cures. With this approach, the fluid can be circulated through the mold substantially all the time that the mold is being used to make successive replications of the object being molded.
Methods and devices for controlling the temperature of a fluid-cooled injection mold without the need for a continuous flow of cooling fluid are described in U.S. Pat. Nos. 4,354,812 and 4,420,446 to Horst K. Wieder, et al. These patents describe methods by which an injection mold can be maintained at a desired operating temperature using a cooling fluid which need not be elevated to or maintained at an ideal operating temperature. Accurate control of the temperature of an injection mold can be achieved by mounting a temperature sensor onto or within the mold. The temperature sensor provides an output signal indicative of the mold temperature. If the sensed mold temperature exceeds a selected control temperature level, a valve is opened to allow cooling fluid to enter the cooling channels in the mold, to thereby cool the mold. When the temperature sensor indicates that the mold is cooled below the control temperature, the valve is closed. Since cooling fluid is not continuously pumped through the mold cooling channels, the cooling fluid need not be heated to a particular operating temperature and the consumption of cooling fluid is reduced.
Another method of injection mold temperature control is described in U.S. Pat. No. 5,427,720 to Kotzab. Typically, a plurality of cooling channels are formed in an injection mold to provide cooling fluid to the mold. This patent describes determining, empirically or by calculation, a selected distribution profile for distributing cooling fluid among the cooling channels to achieve the desired amount of cooling of the injection mold. Depending upon the shape of the object being molded, certain portions of the injection mold may require more cooling than others. At the same time during each molding cycle, a temperature sensor signal is used to determine the temperature deviation of the mold from a desired temperature. Simultaneously, valves are opened to provide pulses of cooling fluid through the cooling channels in the pre-determined distribution profile. The duration of the cooling pulses is determined by the measured temperature deviation.
For some applications, the “pulse” cooling injection mold temperature control schemes just described may employ ordinary tap water as the cooling fluid. However, for many molding operations, the operating temperature of an injection mold can be very high. For example, the operating temperature of an injection mold for a high temperature molding process may be 300 F or higher, and the molten material injected into the mold may typically be at 700 F or higher. Water may be unsuitable as a cooling fluid for such high temperature molding operations without additional measures, such as pressurization, since water at normal atmospheric pressure will instantly turn to steam upon entering the cooling fluid channels of such a high temperature injection mold.
Petroleum-based oils or synthetic heat transfer fluids have been employed as cooling fluids for controlling the mold temperature of high temperature injection molding operations. The use of such materials for high temperature injection mold cooling has several important limitations, however. Such fluids have an inherently poor heat transfer rate. Thus, the time needed during a production cycle to bring the injection mold to the desired operating temperature using such fluids is relatively long, thereby increasing the cycle duration, and decreasing the production rate.
Furthermore, petroleum based oils are difficult to work with and potentially dangerous. The combination of petroleum-based oil and high temperatures presents a fire hazard. The use of oil-based cooling fluids can also adversely affect the quality of a molded object. Hydrocarbon molecules from the cooling oil can get into the mold itself. These molecules will leave flow marks on the molded plastic or metal object. These flow marks can adversely affect the quality and appearance of the molded object. In particular, flow marks on an aluminum die cast object, caused by oil based cooling fluid contamination, will prevent finishing of the aluminum object in the affected area. If the die cast aluminum object cannot be finished properly, it must typically be scrapped or recycled.